1. Field of the Invention
The present invention relates generally to a signal detecting device and method for a CDMA communication system, and in particular, to a device and method for acquiring a signal on a reverse link to perform multipath detection and finger assignment.
2. Description of the Related Art
FIG. 1 illustrates signals transmitted over a reverse link in a mobile station for CDMA (Code Division Multiple Access) mobile communication systems. A pilot channel and a traffic channel are shown in the reverse link of FIG. 1. The pilot channel for the reverse link is transmitted at a constant transmission power. Although transmission power of the reverse pilot channel can be changed according to power control, there is rarely any variation in the ratio of the total transmission power to the pilot power.
A base station despreads a signal on a reverse link transmitted from a mobile station and thereafter, acquires a multipath component to assign a finger. At this point, the base station establishes a specific window to search signals for the window duration. However, in an IMT-2000 CDMA communication system having a wide bandwidth, since a receiver has a high time resolution, the window to be searched by a searcher increases in chip size. In addition, energy received over one path can decrease as the time resolution increases, as compared with a conventional narrow band communication system. Increasing the ratio of the total transmission power to the pilot power because of the increased bandwidth imposes a heavy burden on the searcher in performing the search process. Therefore, there is a limitation in reducing transmission power of the pilot channel.
When only a fundamental channel of 9600 bps is transmitted in a reverse link for the IMT-2000 CDMA communication system, 20-25% of the total transmission power of the mobile station is assigned to a pilot channel. Since this transmission power is not enough for the searcher in a base station to search the transmitted pilot channel, the searcher in the base station calculates a correlation value for a long integrating time to detect a multipath component. However, calculating the correlation value for a long integrating time causes an increase in the complexity of the hardware and increased power consumption. In addition, when the mobile station performs a handoff, a base station attempting connection to the mobile station should acquire a signal from the mobile station. However, since the signal strength or the signal on the pilot channel from the mobile station is too low, the base station will experience difficulty in acquiring the signal from the mobile station.
When the pilot signal transmitted over the reverse link has a low signal strength, the searcher of the base station should despread an input signal for a long time to detect the signal. Further, in order to rapidly acquire the signal with accuracy, the base station requires a searcher with a complicated hardware structure.
It is, therefore, an object of the present invention to provide a device and method for effectively acquiring a multipath component of a reverse link to assign a finger in a CDMA communication system.
It is another object of the present invention to provide a device and method for acquiring, at a base station, a signal transmitted from a mobile station and a multipath signal in a CDMA communication system.
It is a further object of the present invention to provide a device and method for accurately measuring transmission power or time delay of a reverse link signal transmitted from a mobile station in a CDMA mobile communication system.
It is still another object of the present invention to provide a device and method for decreasing pilot power in normal time and instead, increasing the pilot power for a predetermined short time at a mobile station, to increase the overall system capacity for a reverse link, in a CDMA communication system.
To achieve the above objects, a mobile station according to the present invention transmits a pilot signal, a common channel signal and a traffic channel signal for a reverse link. The mobile station normally transmits the common channel signal and the pilot signal at predetermined transmission power, and transmits the pilot signal at a transmission power higher than the predetermined transmission power for a predetermined duration.
Preferably, the pilot signal transmitted at the predetermined duration is spread with a first spreading code over the duration, where the pilot signal is transmitted at the predetermined transmission power, and is spread with a second spreading code over the duration where the pilot signal is transmitted at a transmission power higher than the predetermined transmission power.
Alternatively, the pilot signal transmitted over the predetermined duration is spread with a first spreading code over the duration where the pilot signal is transmitted at the predetermined transmission power, and is spread with at least two spreading codes including the first spreading code over the duration where the pilot signal is transmitted at the transmission power higher than the predetermined transmission power.
Preferably, the spreading code is an orthogonal code and the predetermined duration covers a boundary of data frames for the traffic channel signal.